1. Technical Field
The present invention relates to systems and processes for conducting hydrocarbon synthesis and to cobalt-on-alumina catalysts employed in such processes.
2. Background
In Fischer-Tropsch processes, a synthesis gas ("syngas") comprising carbon oxide(s) and hydrogen is reacted in the presence of a Fischer-Tropsch catalyst to produce liquid hydrocarbons. Certain advanced cobalt catalysts have proven to be very effective for Fischer-Tropsch synthesis. However, for these catalysts, extensive promotion with noble and/or near noble metals has been required in order to enhance the reducibility of the cobalt to an extent sufficient to achieve acceptable Fischer-Tropsch conversion activities. Due in significant part to the cost of obtaining and adding such promoters, these cobalt catalysts have typically been quite expensive. Thus, a need presently exists for a means of significantly reducing the cost of cobalt catalysts useful for Fischer-Tropsch synthesis while maintaining activity levels which are at least comparable to those heretofore obtained by promoting such catalysts with noble metals.
The "syngas" employed in Fischer-Tropsch processes can be produced, for example, during coal gasification. Processes are also well known for obtaining syngas from other hydrocarbons, including natural gas. U.S. Pat. No. 4,423,265 to Chu et al. notes that the major processes for producing syngas depend either upon (a) the partial combustion of the hydrocarbon fuel with an oxygen-containing gas, (b) the reaction of a hydrocarbon fuel with steam, or (c) a combination of these two reactions. U.S. Pat. No. 5,324,335 to Benham et al. explains the two primary methods (i.e., steam reforming and partial oxidation) for producing syngas from methane. The Encyclopedia of Chemical Technology, Second Edition, Volume 10, pages 3553-433 (1966), Interscience Publishers, New York, N.Y. and Third Edition, Volume 11, pages 410-446 (1980), John Wiley and Sons, New York, N.Y. is said by Chu et al. to contain an excellent summary of gas manufacture, including the manufacture of synthesis gas.
It has long been recognized that syngas can be converted to liquid hydrocarbons by the catalytic hydrogenation of carbon monoxide. The general chemistry of the Fischer-Tropsch synthesis process is as follows: EQU CO+2H.sub.2.fwdarw.(--CH.sub.2 --)+H.sub.2 O (1) EQU 2CO+H.sub.2.fwdarw.(--CH.sub.2 --)+CO.sub.2 (2)
The types and amounts of reaction products, i.e., the lengths of carbon chains, obtained via Fischer-Tropsch synthesis can vary depending upon process kinetics and choice of catalyst.
Many attempts at providing effective catalysts for selectively converting syngas to liquid hydrocarbons have been disclosed. U.S. Pat. No. 5,248,701 to Soled et al., presents an over-view of relevant prior art. The two most popular types of catalysts heretofore used in Fischer-Tropsch synthesis have been iron-based catalysts and cobalt-based catalysts. U.S. Pat. No. 5,324,335 to Benham et al. discusses the fact that iron-based catalysts, due to their high water gas shift activity, favor the overall reaction shown in (2) above, while cobalt-based catalysts tend to favor reaction scheme (1).
The current practice is to support the catalytic components on porous, inorganic refractory oxides. Particularly preferred supports have included silica, alumina, silica-alumina, and titania. In addition, other refractory oxides from Groups III, IV, V, VI and VIII have been used as catalyst supports.
As mentioned above, the prevailing practice has been to also add promoters to the supported catalysts. Promoters have typically included noble metals, such as ruthenium, and near noble metals. Promoters are known to increase the activity of the catalyst, sometimes rendering the catalyst three to four times as active as its unpromoted counterpart. Unfortunately, effective promoter materials are typically quite costly both to obtain and to add to the catalyst.
Contemporary cobalt catalysts are typically prepared by impregnating the support with the catalytic material. As described in U.S. Pat. No. 5,252,613 to Chang et al., a typical catalyst preparation may involve impregnation, by incipient wetness or other known techniques, of, for example, a cobalt nitrate salt onto a titania, silica or alumina support, optionally followed or preceded by impregnation with a promoter material. Excess liquid is then removed and the catalyst precursor is dried. Following drying, or as a continuation thereof, the catalyst is calcined to convert the salt or compound to its corresponding oxide(s). The oxide is then reduced by treatment with hydrogen, or a hydrogen-containing gas, for a period of time sufficient to substantially reduce the oxide to the elemental or catalytic form of the metal. U.S. Pat. No. 5,498,638 to Long points to U.S. Pat. Nos. 4,673,993, 4,717,702, 4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, and 5,292,705 as disclosing well known catalyst preparation techniques.
Fischer-Tropsch synthesis has heretofore been primarily conducted in fixed bed reactors, gas-solid reactors, and gas-entrained fluidized bed reactors, fixed bed reactors being the most utilized. U.S. Pat. No. 4,670,472 to Dyer et al. provides a bibliography of several references describing these systems.
Recently, however, considerable efforts have been directed toward conducting Fischer-Tropsch synthesis in three-phase (i.e., solid, liquid, and gas/vapor) reactors. One such system is the slurry bubble column reactor (SBCR). In a SBCR, catalyst particles are slurried in liquid hydrocarbons within a reactor chamber, typically a tall column. Syngas is then introduced at the bottom of the column through a distributor plate, which produces small gas bubbles. The gas bubbles migrate up and through the column, causing a beneficial turbulence, while reacting in the presence of the catalyst to produce liquid and gaseous hydrocarbon products. Gaseous products are captured at the top of the SBCR, while liquid products are recovered through a filter which separates the liquid hydrocarbons from the catalyst fines. U.S. Pat. Nos. 4,684,756, 4,788,222, 5,157,054, 5,348,982, and 5,527,473 reference this type of system and provide citations to pertinent patent and literature art.
It is recognized that conducting Fischer-Tropsch synthesis using a SBCR system could provide significant advantages over the reaction systems commonly employed heretofore. As noted by Rice et al. in U.S. Pat. No. 4,788,222, the potential benefits of a slurry process over a fixed bed process include better control of the exothermic heat produced by the Fischer-Tropsch reactions as well as better maintenance of catalyst activity by allowing continuous recycling, recovery and rejuvenation procedures to be implemented. U.S. Pat. Nos. 5,157,054, 5,348,982, and 5,527,473 also discuss advantages of the SBCR process. However, the slurry bubble column process has been expensive to operate, owing in part to the significant catalyst costs required.